Solubilities of (2,5-dihydroxyphenyl)diphenyl phosphine oxide in selected solvents

Article abstract of DOI:10.1021/je900730w

A phosphorus-containing flame retardant (2,5-dihydroxyphenyl)diphenyl phosphine oxide (HPO) was characterized by a differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), elemental analysis, and nuclear magnetic resonance (¹H NMR and ³¹P NMR). Using a static analytical method, the solubilities of HPO were measured in 12 solvents and correlated with an empirical equation. The estimated uncertainty of all of the solubility values on error analysis and repeated observations was within 2.0 %.

Full text of DOI:10.1021/je900730w

J. Chem. Eng. Data 2010, 55, 1745–1749
1745
Solubilities of (2,5-Dihydroxyphenyl)diphenyl Phosphine Oxide in Selected
Solvents
Xiu-Zhi Guo,† Li-Sheng Wang,* and Na-na Tian
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
A phosphorus-containing flame retardant (2,5-dihydroxyphenyl)diphenyl phosphine oxide (HPO) was
characterized by a differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), elemental
analysis, and nuclear magnetic resonance (1H NMR and 31P NMR). Using a static analytical method, the
solubilities of HPO were measured in 12 solvents and correlated with an empirical equation. The estimated
uncertainty of all of the solubility values on error analysis and repeated observations was within 2.0 %.
Introduction
Epoxy resins are high-performance thermosetting resins that
have excellent properties including high adhesion to many
substrates, chemical and heat resistance, low shrinkage on cure,
toughness, superior electrical and mechanical properties, and
good dimensional stability.1-3 Furthermore, almost any property
of epoxy can be modified to meet a specific need. To extend
the applications of epoxy resins as electronic materials and in
the aerospace industry, it is crucial to improve their thermal
Figure 1. Structures of (2,5-dihydroxyphenyl)diphenyl phosphine oxide
and flame resistance. Several approaches have been reported in
the literature for the improvement of flame resistance. With
safety and environmental concerns in mind, epoxy resins that
are flame-resistant and halogen-free have been the focus of
attention by researchers in recent years.4,5 Among nonhaloge-
nated flame retardants, phosphorus-containing compounds are
attractive owing to their low generation of smoke and toxic gases
and the high flame-retardant efficiency.6
The most effective way to improve flame retardancy in epoxy
resin is the reactive approach: incorporating phosphorus-
containing chemical units into the polymer backbone or side
chain. Thus, while flame retardancy is increased, the original
physical and mechanical properties are maintained. Organo-
phosphorus flame retardants such as 2-(6-oxido-6H-dibenz
[c,e][1,2]oxaphosphorin-6-yl)-1,4-dihydroxyphenylene (ODOPB;
its formula is shown in Figure 1) have been widely used for
this purpose. Owing to the introduction of the rigid structure of
ODOPB and the pendant phosphorus-containing group, the
resultant resins provided not only better flame retardant proper-
ties but also a higher thermal stability and glass transition
temperature.7
(HPO) and 2-(6-oxido-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)-1,4-dihy-
droxyphenylene (ODOPB).
Both ODOPB and HPO were obtained by a subsequent
crystallization upon cooling the reaction mixture and washing
the filter cake with ethanol.8 The solubility data of ODOPB in
selected solvents were reported by Fan et al.10 Knowledge of
solubilities of HPO in toluene and ethanol (as well as other
common solvents such as methanol, 2-ethoxyethanol, and
acetone) as a function of temperature is necessary for subsequent
purification.
In our continuous efforts to search for high thermally stable
flame-retardant polyesters, HPO was synthesized and character-
ized. The solubility data of HPO in 2-ethoxyethanol, methanol,
ethanol, toluene, and acetone were measured. In the purification
of HPO, it was found that the solid can be washed by water
instead of toluene to remove the unreacted p-benzoquinone. The
solubilities of HPO in 2-ethoxyethanol + water binary mixtures
were also measured. To the best of our knowledge, no such
data have been reported in the literature.
An aromatic phosphorus-containing diol is (2,5-dihydrox-
yphenyl)diphenyl phosphine oxide (here after abbreviated as
HPO,8,9 and its formula is shown in Figure 1) has a CAS RN
13291-46-8. It is well-known that these reactive additives are
useful for modifying epoxy resins. HPO was prepared by the
reaction of diphenylphosphine oxide (DPPO) and excessive
p-benzoquinone in toluene. The ODOPB was prepared via a
similar process by reaction of 9,10-dihydro-9-oxa-10-phos-
phaphenanthrene-10-oxide (DOPO) and excessive p-benzo-
quinone. However, DOPO is more expensive than DPPO.
Experimental Section
Materials. HPO was synthesized and provided by Shandong
Wan Zhao Co., Ltd. All of the chemicals were analytical grade
reagents, which were purchased from Beijing Chemical Factory.
They were used without further purification. Table 1 presents
some physical properties of solvents such as density, refractive
index, and purity. Their mass fraction purities were all higher
than 0.99. The water was deionized before use.
Apparatus and Procedure. The melting temperature and
enthalpy of fusion were determined with a DSC Q100 dif-
ferential scanning calorimeter (DSC) in flowing nitrogen at a
heating rate of 10 K ·min-1. The elemental analysis was
* Corresponding author. Fax: +86-10-68911040. E-mail: lishengwang@
btamail.net.cn.
† Current address: Hulunbeier University, People’s Republic of China.
10.1021/je900730w CCC: $40.75  2010 American Chemical Society
Published on Web 11/13/2009

1746 Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010
Figure 3. Experimental heat Q flow from differential scanning calorimeter
(DSC) measurement of HPO.
Figure 2. Schematic diagram of the experimental apparatus: 1, thermo-
couple; 2, sample gauge; 3, rubber plug; 4, jacket; 5, equilibrium cell; 6,
magnetic stirrer; 7, water cycling bath.
Table 1. Mass Fraction Purity (ω), Density (G), and Refractive
Index (nD) for the Organic Solvents Used in This Work at T )
293.15 K
solvent
methanol
ethanol
2-ethoxyethanol 99.5
toluene
acetone
100 ω F/g·cm-3 lit.16 F/g·cm-3
nD
lit.16 nD
99.5
99.7
0.792
0.790
0.929
0.866
0.790
0.79104
0.78920
0.92945
0.86683
0.78998
1.3301 1.32840
1.3660 1.36143
1.4065 1.4077
1.4967 1.49693
1.3590 1.35868
99.5
99.5
performed on an Elementar Vario EL element analyzer, and 1H
NMR and 31P NMR spectra were obtained with a Bruker ARX-
400 and JEOL ECA-600, respectively. Thermogravimetric
analysis (TGA) was carried out with an SDT Q600 thermo-
gravimetric analyzer at a heating rate of 10 K·min-1 under
nitrogen from (298.15 to 1073.15) K.
Figure 4. Experimental heat flow from thermogravimetric analysis (TGA)
measurement of HPO in flowing nitrogen.
Solubility Measurement. The solubilities were measured by
a gravimetric method.11 For each measurement, an excess mass
of HPO was added to a known mass of solvent. Then the
equilibrium cell was heated to a constant temperature with
continuous stirring. After at least 2 h (the temperature of the
water bath approached a constant value, and then the actual
value of temperature was recorded), the stirring was stopped,
and the solution was kept still until it was clear. A preheated
on-off injector withdrew 2 mL of the clear upper portion of
the solution to another previously weighed measuring vial (m0).
The vial was quickly and tightly closed and weighed (m1) to
determine the mass of the sample (m1 - m0). Then the vial was
uncovered with a piece of filter paper to prevent dust contami-
nation. After the solvent in the vial had completely evaporated,
the vial was dried and reweighed (m2) to determine the mass of
the constant residue solid (m2 - m0). Thus, the solid concentra-
tion of the sample solution in mole fraction, x, could be
determined from eq 111
The setup for the solubility measurement was the same as
that described in the literature.11,12 Figure 2 shows the schematic
diagram of the experimental apparatus. A jacketed equilibrium
cell was used for the solubility measurement with a working
volume of 120 mL and a magnetic stirrer, and a circulating water
bath was used with a thermostat (type 50 L, made from Shanghai
Laboratory Instrument Works Co., Ltd.), which is capable of
maintaining the temperature within ( 0.05 K. An analytical
balance (type TG328B, Shanghai Balance Instrument Works
Co.) with an uncertainty of ( 0.1 mg was used during the mass
measurements.
1
Characterization of HPO. H NMR (DMSO-d6), δ (ppm):
6.73 (d, 1H), 6.87 (d, 1H), 6.95 (d, 1H), 7.50 (m, 4H), 7.58 (m,
2H) 7.73 (m, 4H), 9.13 (s, 1H), 9.78 (s, 1H), [lit.9 1H NMR
(500 MHz, CDCl3) δ (ppm): 5.05 (s 1H,), 6.43 (d, 1H), 6.85
(d, 1H), 6.91 (d, 1H), 7.44 to 7.50 (m, 4H), 7.55 to 7.60 (m,
2H), 7.63 to 7.69 (m, 4H), 10.51 (s, 1H); lit.13 1H NMR
(CD3OD/TMS, δ (ppm): 5.3 (s, 2H), 6.72 (d, 1H), 6.87 (d, 1H),
6.92 (d, 1H), 7.52 (m, 4H), 7.57 (m, 2H), 7.70 (m, 4H)], 31P
NMR (DMSO-d6): δ ) 33.99 ppm [lit.9 1H NMR (500 MHz,
CDCl3) (101.3 MHz MeOH, external D2O lock) δ ) 30.1 ppm;
lit.13 31P NMR (CD3OD/H3PO4) δ ) 34.09 ppm]. Elemental
analysis (%, calcd): C, 69.06 % (69.7 %); H, 5.01 % (4.9 %).
On the basis of the above analysis, the mass fraction purity of
HPO used in this work was higher than 0.99. The results of
DSC and TGA measurements of HPO are shown in Figures 3
and 4. The melting temperature of HPO was 487.75 K (lit.9,13
(487.15 to 489.15) K; lit.8 (489.15 to 491.15) K). The enthalpy
of fusion of HPO was 37.26 kJ ·mol-1. TGA results show that
there is a single-step decomposition, and no or very little residue
remains for HPO.
(m2 - m0)/M1
(m2 - m0)/M1 + (m1 - m2)/M2
x )
(1)
where M1 is the molar mass of HPO and M2 is the molar mass
of solvent.
x )
(m2 - m0)/M1
(m2 - m0)/M1 + (m1 - m2)w1/M2 + (m1 - m2)(1 - w2)/M3
(2)
Equation 2 is for the mixed solvent, where M1, M2, and M3
are the molar masses of HPO, 2-ethoxyethanol, and water and
w2 is the mass fraction of 2-ethoxyethanol in the solvents.

Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010 1747
Table 2. Mole Fraction Solubilities (x) and Activity Coefficients (γ)
ln x ) A + B/(T/Κ)
(3)
of HPO in the Selected Solvents
Parameters A and B for each solvent are listed in Table 4. The
relative standard deviations (RSD), defined by eq 4, are also
presented in Table 4. The smoothed data calculated from eq 3
are compared with the data listed in Tables 2 and 3.
solvent
toluene
T/K
x
γ
(x - xcalcd)/x
-0.00266
0.00552
-0.00296
0.00416
-0.00201
-0.00478
-0.00224
0.00487
0.01143
-0.00328
0.00354
-0.01521
-0.02038
-0.00798
0.02099
0.00501
0.00336
293.96
298.25
303.47
308.24
313.45
318.43
323.38
328.45
299.45
303.26
308.24
313.47
318.13
323.26
328.25
294.16
298.35
303.87
308.85
313.34
318.15
323.56
293.95
298.26
303.44
308.26
313.45
318.47
323.35
328.43
333.35
297.16
308.15
313.34
318.15
323.47
328.46
333.25
338.17
343.25
348.36
353.14
358.13
363.27
368.26
299.43
303.25
308.24
313.46
318.15
323.27
328.26
333.45
338.75
343.15
348.15
353.14
358.06
363.15
368.16
0.00071
0.00075
0.00078
0.00082
0.00086
0.00089
0.00094
0.00099
0.00175
0.00188
0.00216
0.00236
0.00262
0.00296
0.00332
0.00625
0.00707
0.00814
0.00946
0.01082
0.01232
0.01383
0.00571
0.00634
0.00721
0.00787
0.00903
0.00979
0.01093
0.01168
0.01267
0.00011
0.00012
0.00013
0.00014
0.00014
0.00015
0.00015
0.00016
0.00017
0.00017
0.00018
0.00018
0.00019
0.00019
0.01076
0.01131
0.01241
0.01367
0.01544
0.01681
0.01803
0.01983
0.02229
0.02356
0.02521
0.02786
0.02955
0.03158
0.03482
3.2968
3.8863
4.8388
5.7846
7.0233
8.4871
9.9675
11.721
1.7689
1.9872
2.1961
2.5617
2.8450
3.1491
3.4663
0.3784
0.4143
0.4727
0.5160
0.5554
0.6056
0.6827
0.4097
0.4599
0.5227
0.6033
0.6689
0.7729
0.8561
0.9927
1.11937
25.076
39.361
46.229
53.289
67.183
77.394
94.163
107.36
122.94
148.88
167.36
199.73
225.87
266.97
0.2874
0.3301
0.3822
0.4421
0.4832
0.5547
0.6385
0.7181
0.7883
0.8836
0.9962
1.0813
1.2136
1.3534
1.4519
xi - xci alcd
2
1/2
n
1
N
RSD )
(4)
∑
(
)
[
]
xi
1
acetone
methanol
ethanol
where calcd stands for the calculated values and N is the number
of experimental points. The results show that eq 3 can be used
Table 3. Mole Fraction Solubilities (x) and Activity Coefficients (γ)
of HPO in w 2-Ethoxyethanol + (1 - w) Water
w
T/K
x
γ
(x - xcalcd)/x
-0.01369
-0.00136
0.01083
0.1983
299.85
303.46
308.14
313.65
318.37
323.25
328.23
333.55
338.16
343.14
348.15
353.86
358.35
363.24
368.35
298.25
303.36
308.07
313.44
318.56
323.15
328.14
333.54
338.85
343.16
348.15
353.34
358.06
363.35
368.26
299.85
303.46
308.14
313.65
318.37
323.25
328.23
333.54
338.15
343.16
348.15
353.84
358.35
363.26
368.35
298.25
303.36
308.05
313.44
318.55
323.16
328.15
333.54
338.85
343.17
348.13
353.35
358.06
363.34
368.25
0.00017
0.00018
0.00019
0.00022
0.00024
0.00027
0.00028
0.00031
0.00033
0.00036
0.00038
0.00041
0.00044
0.00048
0.00051
0.00025
0.00027
0.00029
0.00032
0.00035
0.00037
0.00040
0.00042
0.00044
0.00047
0.00051
0.00053
0.00056
0.00061
0.00063
0.00119
0.00131
0.00147
0.00165
0.00191
0.00214
0.00237
0.00269
0.00298
0.00339
0.00373
0.00415
0.00461
0.00501
0.00547
0.00519
0.00571
0.00659
0.00699
0.00795
0.00858
0.00951
0.01068
0.01153
0.01251
0.01351
0.01477
0.01548
0.01735
0.01831
18.576
20.958
24.847
27.706
31.389
34.508
41.066
46.117
52.031
57.811
66.091
75.397
82.338
89.317
99.756
11.658
13.904
16.225
18.866
21.706
25.074
27.941
34.025
40.091
44.313
49.244
57.249
64.042
70.545
80.515
2.6537
2.8797
3.2116
3.6941
3.9442
4.3539
4.8517
5.3125
5.7595
6.1438
6.7332
7.4435
7.8588
8.5631
9.3008
0.5615
0.6574
0.7133
0.8637
0.9551
1.0817
1.2051
1.3381
1.5299
1.6655
1.8576
2.0550
2.3168
2.4794
2.7694
0.01035
-0.02511
-0.00269
0.00168
0.01898
0.02158
0.01527
-0.00539
-0.01267
-0.00522
0.00802
-0.00722
0.02267
0.00469
0.01838
-0.01055
-0.01892
-0.01138
-0.01069
0.00873
0.00175
0.00339
0.01142
0.00721
0.00342
0.00607
0.00031
-0.00381
-0.00906
-0.00039
-0.00734
0.02552
-0.00474
-0.01355
-0.01885
0.01363
0.00376
-0.02053
-0.00055
-0.00237
-0.00489
-0.02089
0.00021
0.00832
0.01089
0.00368
0.00141
-0.02034
-0.00097
0.01441
0.00957
0.01965
-0.00999
-0.02139
0.00013
-0.00067
-0.00153
-0.00868
0.00896
water
0.4012
0.6032
0.8043
2-ethoxyethanol
0.00372
0.00489
0.01519
0.00987
-0.015132
-0.01016
0.01736
0.00073
-0.01144
0.01126
-0.00341
-0.01142
0.80669
-0.00046
-0.02246
0.00235
-0.00104
-0.01512
-0.00991
-0.00741
0.01461
Different dissolution times were tested to determine a suitable
equilibrium time. It was found that 2 h was enough to reach
equilibrium. During our experiments, three parallel measure-
ments were performed at the same composition of solvent for
each temperature, and an average value is given. The maximum
RSD of each triplicate data is 0.24 %, and the minimum is 0.12
%. The estimated relative uncertainty of the mole fraction
solubility values based on error analysis and repeated observa-
tions was within 0.02.
0.00457
-0.00239
0.01329
0.00178
-0.00365
-0.00299
-0.00747
0.03531
-0.02484
0.00520
-0.01987
0.00379
0.01994
-0.00261
-0.00098
-0.00319
0.00616
Results and Discussion
The mole fraction solubilities x of HPO in selected solvents
are summarized in Tables 2 and 3 and plotted as ln x versus T
in Figures 5 and 6. From these figures, it can be seen that a
trend of increasing solubility with temperature is observed.
The solubilities were correlated as a function of temperature
by
-0.02682
0.00794
0.00164

1748 Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010
Figure 5. Mole fraction solubilities of HPO in: 9, 2-ethoxyethanol; O,
methanol; 2, ethanol; 3, acetone; b, toluene; ], water (experimental); solid
line, solubility curve calculated from eq 3.
Figure 7. Solubility data of 9, HPO; 2, ODOPB, and b, p-benzoquinone;
b, dotted line, data from ref 15; 2, data from ref 10; 9, experimental data
obtained in this work; solid line, dashed line, solubility curve calculated
from eq 3.
658.2 K. Figure 3 shows the results of the DSC measurement
of HPO. The enthalpy of fusion of HPO was 37.26 kJ ·mol-1
,
and ODOPB10 was 41.70 kJ ·mol-1. The melting temperature
of HPO was 487.75 K; ODOPB10 was T ) 524.07 K. The
results indicate that the HPO has better flame retardancy for
epoxy resin like the ODOPB.
In the preparation of HPO, an excess amount of p-benzo-
quinone (as compared with the stoichiometric amount) is needed.
The p-benzoquinone is the most likely impurity in the reaction
mixture. The separation of the p-benzoquinone from HPO can
be achieved by washing the reaction mixture with warm water.
The solubility data of HPO and p-benzoquinone (the mass in
100 g of solvent), c, in the water are plotted versus T in Figure
7. In Figure 7, it can be shown that evidence of using water in
the first stage of purification for HPO is obvious. The solubility
data of HPO in water from (303.00 to 342.35) K is lower than
those of ODOPB derived from the literature data10 for the sake
of comparison; 2-ethoxyethanol is recommended as the best
solvent for the recrystallization of HPO as the second stage of
purification because of its higher boiling temperature and good
dissolubility with HPO. For the final stage of purification, water
is recommended as the solvent to remove the 2-ethoxyethanol
from the slurry by quickly filtrating and drying.
Figure 6. Mole fraction solubilities of HPO in w 2-ethoxyethanol + (1 -
w) water: 3, w ) 1; b, w ) 0.8043; ], w ) 0.6032; 2, w ) 0.4012; O,
w ) 0.1983; 9, w ) 0; solid line, solubility curve calculated from eq 3.
Table 4. Parameters of Equation 3 and Root-Mean-Square
Deviations of the Measured Solubility Calculated from Equation 4
for w 2-Ethoxyethanol + (1 - w) Water, Acetone, Methanol,
Toluene, and Ethanol
solvent
w ) 0
w ) 0.1983
w ) 0.4012
w ) 0.6032
w ) 0.8043
w ) 1
toluene
acetone
methanol
ethanol
A
B
RSD
-5.86324
-2.74466
-3.46523
1.5485
1.41434
1.79126
-4.14133
0.91846
3.77077
1.64716
-976.57
-1780.71
-1439.55
-2487.64
-1991.43
-1901.11
-913.61
-2179.25
-2603.37
-1999.04
0.00773
0.01428
0.01193
0.01126
0.01445
0.01437
0.00446
0.01343
0.00986
0.01538
To obtain the activity coefficients of HPO in the solvents
from the experimental data, the following equilibrium equation
for solute 1 was derived as a fair approximation14
∆Hfus Tm
1
x1γ1
ln
)
- 1
(5)
(
)
RTm
T
where ∆fusH refers to the enthalpy of fusion, Tm is the melting
temperature, R is the gas constant, and x1 and γ1 refer to the
mole fraction and activity coefficient of solute in the solution,
respectively. With the experimental x1, T, ∆fusH, and Tm values
known, the activity coefficients of HPO in different solvents
were obtained. The results are listed in Tables 2 and 3. From
Tables 2 and 3, it can be seen that the activity coefficients of
HPO in 2-ethoxyethanol, methanol, ethanol, and acetone are
all less than unity. A relatively higher solubility in those solvents
than the ideal behavior which corresponds to a polar or specific
chemical force is important.
to correlate the solubility data. Within the temperature range
of the measurements, the solubilities of HPO in all of the
investigated solvents increased with an increase in temperature.
The solubility of HPO in water shows the lowest value and in
2-ethoxyethanol shows the highest value from (299.43 to 368.16)
K. This result is similar to that for the solubilities of ODOPB
obtained by Fan et al.10
The thermogravimetric physical property of HPO and ODOPB
is similar. The thermogravimetric curve of HPO is shown in
Figure 4. The initial decomposition temperature of HPO was
around T ) 575.72 K, and ODOPB10 was around T ) 605.9
K. The temperature at 97.4 % mass loss was T ) 650.59 K,
and the temperature of ODOPB10 at 93.4 % mass loss was T )
Supporting Information Available:
1H NMR and 31P NMR spectra of HPO information. This material
is available free of charge via the Internet at http://pubs.acs.org.

Journal of Chemical & Engineering Data, Vol. 55, No. 4, 2010 1749
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Received for review September 6, 2009. Accepted October 21, 2009.
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